Experimental Observation of Disorder-Driven Hysteresis-Loop Criticality

We have studied the effect of magnetic disorder on the magnetization reversal process in thin Co/CoO films. The antiferromagnetic CoO layer allows a reversible tuning of the magnetic disorder by simple temperature variation. For temperatures above a critical temperature Tc, we observe a discontinuous magnetization reversal, whereas smooth magnetization loops occur for T<Tc. Our measurements establish the existence of a disorder-driven critical point in the nonequilibrium phase diagram. In addition, we observe scaling behavior in the vicinity of the critical point and determine the critical exponents β0ex0ex=0ex0ex0.022±0.006 and βδ0ex0ex=0ex0ex0.30±0.03 for this two-dimensional system.